Variable equalizer



June 6, 1950 R. s. GRAHAM 2,510,233

VARIABLE EQUALIZER Filed Jan. 18, 1949 F/G.8 nwe/vroe s 2 RR By R. S.GRAHAM A 7' TORNEV Patented June 6, 1950 VARIABLE EQUALIZER Robert S.Graham, Bernardsville, N. J assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJanuary 18, 1949, Serial No. 71,413

12 Claims. 1

This invention relates to wave transmission networks and moreparticularly to variable equalizers.

The principal object of the invention is to improve the operation of amultirange variable equalizer of the type requiring only a singlevariable impedance for each operating range. More specific objects areto reduce the interaction between the different operating ranges and todecrease the flat loss of such an equalizer. A further object is toreduce the cost.

In order to compensate for the attenuation distortion in a transmissionline, a variable equalizer of the two-terminal type requiring only asingle variable impedance such, for example, as is disclosed in UnitedStates Patent 2,096,027 issued October 19, 1937, to H. W. Bode may beconnected in series or in shunt with the line. Equalization over twonon-adjacent frequency ranges may be provided by a variable two-terminalequalizer of the type disclosed in United States Patent 2,374,872,issued May 1, 1945, to W. R. Lundry. When more precise equalization thancan be obtained by a single equalizer is required, it is necessary toemploy two or more such equalizers, each of which has a fairly welldefined operating range and which, between them, cover the entire range.

Heretofore, when a plurality of two-terminal equalizers were used inassociation it has been customary to separate them one from another bymeans of interposed resistance pads, or the equivalent, in order toprevent interaction between the difierent equalizers. These'pads notonly increase the number of component elements required, and thereforethe cost of the network, but also increase considerably the flat lossintroduced, which is undesirable. In cases where it is possible toemploy two variable impedances for one operating range, bearing therelation that the products of the impedances for all settings areconstant, it is possible to eliminate the pads by constructing variableequalizers having constant impedance at all frequencies. However, thismethod substantially doubles the complexity of the equalizers, thusincreasing the cost, and also requires somewhat greater flat loss thanthe method herein disclosed.

In accordance with the present invention iso-' lating pads are entirelyeliminated in a composite equalizing network made up of a number oftwoterminal variable equalizers requiring only a single variableimpedance for each operating range. The equalizers are connected inseriesshunt relationship to form a ladder network with the branches soarranged that every pair of adjacent series and shunt equalizers hasnon-adjacent operating ranges. It isalso preferable that adjacentseries-connected equalizers, and adjacent shunt-connected equalizers aswell, should have substantially mutually exclusive operating ranges.

In this way the interaction between the component equalizers is keptwithin permissible limits because, at frequencies sufiiciently far awayfrom the operating range, a two-terminal equalizer has an impedancewhich is substantially constant, regardless of the attenuation setting.Furthermore, since the isolating pads are eliminated, the flat loss isreduced, and also the cost. The proper impedance level for eachequalizer in such a ladder network depends upon the required adjustmentrange of the equalizers, and may be found by trial or may be computed.

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawing, in which like reference characters are used to designatesimilar or corresponding parts and of which:

Fig. 1 is an insertion loss-frequency characteristic showing the limitsof the equalization required over a number of operating ranges;

Fig. 2 is a diagrammatic circuit of a T-type ladder network inaccordance with the invention fOr providing variable equalization overthe operating ranges I, 2 and 5 shown in Fig. 1;

Fig. 3 is a diagrammatic circuit of a 1r-type ladder network equivalentto the one shown in Fig. 2;

Fig. 4 shows a more extensive'ladder network for equalizing the rangesI, 2, 3, 4 and 5;

Fig. 5 shows a ladder network for equalizing all six of the ranges shownin Fig. 1, the equalizer NI 6 operating over the two separated ranges Iand. 6; and 7 Figs. 6, 7 and 8 are schematic circuits representing thenetworks shown, respectively, in Figs. 2, 3 and 4 at frequencies outsideof the range of the individual equalizers, and are referred to inexplaining how the impedance levels of the various branches aredetermined.

The successful operation of long broad-band carrier telephone circuits,or circuits for the transmission of television signals, requires theequalization of large amounts of attenuation distortion with a verysmall permissible tolerance. The daily and seasonal variations in thisdistortion may be many times the tolerances and must be corrected byautomatic or manual adjustments.

The curves in Fig. 1, where insertion loss in decibels is plottedagainst frequency, show the limits of equalization of a typical variableequalizer, designed for such use, providing independent adjustment ofsix different characteristics. Each characteristic has a portion above,and a corresponding portion below, a reference loss A0. The portionsabove An, designated 1 to a, represent insertion loss, as indicated bythe subscripts of the ordinates A1 and A2. The portions below,designated l to 6', represent negative insertion loss, or insertiongain, as indicated by the negative subscripts of the ordinates A-1 andA-2. The oddnumbered characteristics are shown in full line and theeven-numbered ones in broken line to make the'difierent ranges stand outmore clearly. In each range, by the adjustment of a single variableimpedance, there is provided a family of characteristics which arelimited by the curves shown. For example, in the third range the loss atone extreme setting is given by curve'3 and the gain at the opposite"extreme is given by curve 3. The variable impedance may be adjustedmanually or automatically, by means of a pilot channel regulator system,to maintain the circuit gain constant at arbitrarily chosen frequencies.

In Fig. 1, the normal settings of all controls are assumed. to producethe flat or reference loss, A0. If the circuit to be equalized requiresa normal condition which is not flat, such equalization can be providedby equalizers in tandem with the variable equalizer, using well -knowndesign meth ods. The magnitude of A is the amount of gain that isrequired by additional amplification to compensate for the loss of theequalizer. Since the addition of any considerable amount of gain at alloperating frequencies of a broad-band circuit increases the size ornumber of ampli fiers, and hence the cost, possible means of minimizingthe value of A0 are desirable.

The usefulness of a variable equalizer having the characteristics shownin Fig. 1 depends on the property of independent adjustment of thevarious ranges. Changing the setting of any particular operating rangeshould ideally have no efiect on the loss at frequencies removed fromthat range. Practically, the small amounts of this interaction may betolerated but it tends to complicate the adjustment procedure, or, inthe case of automatic regulation, may reduce the stability of thecircuit. Also, some of the adjustment range of the other equalizers isrequired to compensate for this interaction, thus reducing the amount ofrange available for compensating circuit distortion.

Any two-terminal series or shunt variable equalizer producingcharacteristics which are variable over a restricted frequency rangesuch as those in Fig. 1 will have at other frequencies an impedancewhich approaches a value substantially independent of the setting of thevariable control. The approximation is realized to a greater degree atfrequencies further removed from the operating frequency range, and canbe controlled to some extent by design. The present invention takesadvantage of this property of twoterminal equalizers by connecting themin seriesshunt relationship to form a ladder network arranged in amanner to reduce the interaction between equalizers having adjacent orpartly overlapping operating ranges. This is accomlished by making surethat every pair of adja cent series and shunt equalizers hasnon-adjacent operating ranges.

Fig. 2 is a diagrammatic circuit of a threerange equalizing network inaccordance with one embodiment of the invention. The network comprisestwo series equalizers NI and N2 and an interposed shunt equalizer N5connected to form a T. The equalizers are of the two-terminal typerequiring only a single variable impedance for each operating range andhave insertion loss characteristics corresponding, respectively, tothose designated I, 2 and 5 in Fig. 1. The loss may be varied betweenthe limits shown, as indication by the arrows through the boxes. Thecomponent equalizers may, for example, be designed in accordance withthe Bode patent mentioned above. The series equalizers NI and N2 haveoperating ranges which overlap somewhat but the range of the interposedshunt equalizer N5 is non-adjacent to the ranges of the other two and,in fact, is quite far removed from them in frequency. The degree towhich interaction effects between equalizers NI and N2 are reduceddepends upon the amount of loss introduced by equalizer N5. This lossis, in general, somewhat greater than the gain range, shown by curve 5in Fig. l. The greater the magnitude of this loss, the greater thereduction of the interaction effects will be. In the network of Fig. 2interaction between the three different ranges of equalization isnegligible, or at least tolerable.

Fig. 3 is a diagrammatic circuit of another embodiment of the inventionequivalent in performance to the network of Fig. 2 but having relativelylower impedance levels, which may be more desirable for certainapplications. In Fig. 3' the equalizers NI and N2 are of the shunt typeand equalizer N5 is of the series type, interposed between the other twoto form a 11' network. Here, again, it will be noted that each pair ofadjacent series and shunt equalizers has nonadjacent operating ranges.

With a larger number of component equalizers involved, more choice isoffered in Ways of associating them in a ladder network to reduce theundesired interaction effects; For example, Fig. 4 is a diagrammaticcircuit of a five-range equalizing network comprising the componenttwoterminal variable equalizers NI to N5 arranged in an unbalancedstructure. The insertion loss characteristics of equalizers NI to N5correspond, respectively, to those designated I to 5 in Fig. 1.Equalizers N3, N5 and Nd'are arranged in series, in that order, NI isconnected in shunt at the junction of N3 and N5, and N2 is connected inshunt at the junction of N4 and N5. In this network, also, in each ofthe possible pairs of adjacent series and shunt equalizers, namely, NIand N3, NI and N5, N2 and N4, and N2 and N5, the operating ranges arenon-adjacent. Furthermore, the series connected equalizers N3, N4 and.N5 have mutually exclusive operating ranges. Thus, the network of Fig. 4gives a satisfactory performance with respect to interaction between thedifierent ranges.

Fig. 5 is a diagrammaticcircuit of a six-range, unbalanced, laddernetwork in accordance with the invention comprising the componenttwoterminal variable equalizers N2 to N5 and NM. Equalizers N2, NIB andN5 are connected in shunt, in that order, N4 is interposed in seriesbetween N2 and NIG, and N3 is connected in series between N5 and NIB.The insertion loss characteristics of equalizers N2 to N5 correspond,respectively to those designated 2 to 5 in Fig. 1. Equalizer NI6 coversthe two separated ranges I and 6, as indicated by the two arrows, andmay, for example, beof the type disclosed in the abovementioned Lundrypatent. It will be noted that every pair of adjacent series and shuntequalizers has non-adjacent operating ranges. Also, the series-connectedequalizers N3 and N4, as well as the shuntconnected equalizers N5 andN15, have operating ranges which are mutually exclusive. The interactionbetween the different ranges will, therefore, not be troublesome.

Figs. 6, 7 and 8 are schematic circuits representing the networks shown,respectively, in Figs. 2, 3 and 4 at frequencies outside of the range ofthe individual equalizers. The network is shown connected between asource of alternating electromotive force E of impedance Rs and a loadof impedance RR. The component two-terminal equalizers NI to N5 appearas the resistances designated R1 to R5, respectively. The values ofthese resistances, that is, the required impedance levels of theequalizers, may be found by trial or may be computed by the methodoutlined below.

The impedance level, or the impedance the two-terminal equalizer assumesfor the reference control setting, required for each equalizer in any ofthese ladder networks shown in Figs. 2 to 5 can be determined from thearrangement selected for the network, the maximum excursion eachcharacteristic is required to have in the gain direction as shown by thecurves l' to 6 in Fig. 1, and the impedances Rs and RR between which thenetwork must operate. The relation is that the change in impedance ofeach twoterminal equalizer from the reference condition to the maximumgain condition at the frequency of maximum gain must cause a reductionin the insertion loss of the whole network equal to the maximum gainexcursion required. This assumes that, in the condition of maximum gain,the equalizer, if of the series type, presents an impedance of zerovalue at the frequency of maximum gain; and, if of the shunt type,presents an infinite impedance. If this condition is not satisfied, thereduction in the insertion loss must be made correspondingly greater.One such relation holds for each equalizer in the network, and, takentogether, they form a set of simultaneous equations from which theimpedance levels can be computed.

Let An be the ratio of the current flowing in the receiving terminationRR for the maximum gain condition of an equalizer, N, of the seriestype, to the current for the reference condition, assuming the controlsof all the other equalizers remain fixed at their reference conditions,and that the generator voltage, E, remains constant and has thefrequency of maximum gain for equalizer N.

Then, in general, the relation for determining the impedance lever of N,Rn, is

Rn (An1) (Rnl-l-RM) (1) where Rnl and R12 are the two equivalentimpedances facing Rn, toward the generator E and the load Rn,respectively.

Likewise if Am be the corresponding ratio for an equalizer, M, of theshunt type, the relation for determining the impedance level of M, Rm,is

the method of successive approximations:

R R R R Rz=( 2-1)(RR+fi 4) (RH-Rs) (RziRR) (5) In a similar manner a setof equations may be written for determining the values of theresistances R1, R2 and R5 in the 11' network of Fig. 7 and another setfor determining the values of the resistances R1 to R5 in the moreelaborate ladder network shown in Fig. 8. The required empedance levelsof all of the component twoterminal equalizers may thus be determined,since they correspond to the values found for the resistances in theequivalent circuits.

The insertion loss of the ladder network in the reference condition willin all cases be less than the sum of the maximum gain excursions of eachcomponent equalizer. A reduction in the reference loss Ac, as comparedto other known methods of associating variable equalizers in a compositenetwork, is thus effected in the networks of the invention.

What is claimed is:

1'. In combination, a plurality of two-terminal variable equalizersconnected in series-shunt relationship to form a ladder network, everypair of adjacent series and shunt equalizers having non-adjacentoperating ranges.

2. The combination in accordance with claim 1 in which adjacentseries-connected equalizers have substantially mutually exclusiveoperating ranges.

3. The combination in accordance with claim 1 in which adjacentshunt-connected equalizers have substantially mutually exclusiveoperating ranges.

4. The combination in accordance with claim 1 in which said equalizerscomprise only a single variable impedance for each operating range.

5. A. multirange equalizing network comprising two two-terminal variableequalizers connected in series-shunt relationship, said equalizershaving non-adjacent operating ranges.

6. A network in accordance with claim 5 in which said equalizerscomprise only a single variable impedance for each operating range.

7. In combination, three two-terminal variable equalizers, two of saidequalizers being connected in series, the third being connected in shuntat the junction of said two, and said third equalizer having anoperating range which is non-adjacent to the operating ranges of saidtwo equalizers.

8. The combination in accordance with claim 7 in which said twoequalizers have mutually exclusive operating ranges.

9. The combination in accordance with claim '7 in which said equalizerscomprise only a single variable impedance for each operating range.

10. In combination, three two-terminal variable equalizers, two of saidequalizers being connected in shunt at the respective ends of the thirdand said third equalizer having an operating range which is non-adjacentto the operating ranges of said two equalizers.

11. The combination in accordance with claim 10 in which said twoequalizers have mutually exclusive operating ranges.

12. The combination in accordance with claim 10 in which said equalizerscomprise only a single variable impedance for each operating range.

ROBERT S. GRAHAM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,096,027 Bode Oct. 19, 19372,374,872 Lundry May 1, 1945

